Current Automatic Dependent Surveillance (ADS) technology in the form of a Global Positioning System (GPS) provides information utilizing satellite communication. The GPS, developed by the United States Department of Defense, consists of twenty-four (24) satellites that orbit the earth twice a day at an altitude of 12,000 miles. Additionally, the GPS consists of five (5) ground stations that monitor and manage the satellites. Using atomic clocks and location data, GPS satellites transmit continuous time and position information twenty-four (24) hours a day to a GPS receiver, which inputs data from three or more satellites simultaneously to determine the user's position on the earth. By measuring the time interval between the transmission and the reception of a satellite signal, the GPS receiver calculates the distance between the user and each satellite. The GPS receiver then uses the distance measurements from at least three satellites to arrive at a final user position or location.
The civilian GPS measurement yields an accuracy of approximately 100 meters. Additional accuracy, at a level needed for targets such as aircraft, is obtained by using a Differential GPS consisting of ground reference points. However, ground reference stations are expensive to deploy, and their effectiveness is limited to a finite radius from the reference position on the surface of the earth.
The civilian GPS system utilizes one-way ranging in which an accurate, synchronized clock is required at each station. Any synchronization error, or any error regarding the location of one of the satellites, results in an error in the determined position of the target. An aircraft, or other target, must have accurate position and velocity information and requires dedicated, precise equipment throughout the flight or mission to provide accurate position and velocity information with a high degree of integrity and reliability.
To overcome this problem, two-way ranging navigation has been proposed by co-pending U.S. application Ser. No. 08/803,936, which is incorporated herein by reference. The two-way ranging navigation system disclosed therein determines a position of a vehicle, such as an aircraft or surface vehicle, utilizing two-way ranging techniques through multiple satellites to derive independent estimates of a vehicle's state vectors including position and velocity.
The two-way ranging navigation system includes transmitting a first signal to a vehicle from a traffic control station through a plurality of satellites. From that signal, the vehicle acquires and tracks a unique ranging code, and a message signal containing the unique code is transmitted from the vehicle. The links through multiple satellites are sequentially or simultaneously processed to provide two-way ranging with reasonably accurate estimations of state vectors. The two-way ranging navigation system processes ranging measurements to derive estimations of vehicle state vectors in response to a first signal and a message signal at a traffic controller, preferably a ground-based traffic controller employing .alpha.-.beta./EKF (Extended Kalman Factor) tracking.
Two-way ranging navigation can be used as a stand alone positioning system for a target with higher accuracy than GPS systems. During normal communication links, positioning with two-way ranging navigation consumes little space-segment resources. However, when there is no communication between the user and the satellite, also known as "silent intervals", two-way ranging navigation positioning requires additional space-segment bandwidth resources. GPS, on the other hand, being a one-way broadcast positioning system, requires no additional space-segment resources once it is deployed.
Therefore, there is a need to obtain the highest degree of positioning accuracy without the cost of two-way ranging navigation positioning and while utilizing the free civilian GPS signals whenever possible.